The thermoplastic aromatic polyketone polyetheretherketone has been known for over twenty years. EP 0 001 879 (Imperial Chemical Industries) describes its preparation and use. For example, it may be prepared by reaction of 4,4′-difluorobenzophenone and hydroquinone in diphenylsulphone and in the presence of an alkali metal carbonate, for example potassium carbonate and/or sodium carbonate. After the reaction, the diphenylsulphone and inorganic salts are removed by washing successively with acetone (twice), water (twice), and acetone/methanol (twice).
EP 0 292 211 (Imperial Chemical Industries) describes a washing process for removing inorganic salt from an aryl polyether or aryl polythioether phase containing the inorganic salt, wherein the aryl polyether or aryl polythioether phase is initially in the form of a “toffee” and is caused to assume a liquid form by melting and/or dissolution in a solvent, for example diphenyl sulphone. The liquid mixture is contacted with water to dissolve salt out of the mixture.
Polyetheretherketone has been commercially available for many years, initially from Imperial Chemical Industries (the applicant for the abovementioned patent application) and subsequently from Victrex Plc, the assignee of Imperial Chemical Industries. However, notwithstanding attempts to remove inorganic salts from polyetheretherketone in the processes described, commercially available grades of polyetheretherketone (e.g. Victrex standard grade PEEK™ 450 P) tend to have greater than 250 ppm of alkali metal cations, for example Na+, in them.